Question

21. You have taken a job at a laboratory and are tasked with preparing a buffer for the day's experiment. Your first job is to make a buffer that will buffer around a pH of about 7. Choose the best acid for the job from the following choices. Acid К. 7.1 x 10 6.3 x 108 4.2x 10-13 a) Н.РО/ NaН,РО, b) HPO4/HPO2 HPO/PO d) None of these buffers will make an effective buffer around a pH of 7. -as(6.310 c) 7.2 p 4

Answer 1

21.

Correct option: b

Explanation:

Buffering capacity pH range of a buffer solution is given by,

pH = pK_a$\pm$1

      = - logK_a$\pm$1

For H₃PO₄/NaH₂PO₄,

pH = - log(7.1 x 10^-3) $\pm$1

      = 2.1 $\pm$1

The buffering capacity range of H₃PO₄/NaH₂PO₄ is (2.1 - 1) = 1.1 to (2.1 + 1) = 3.1.

For H₂PO₄^-/HPO₄^2-,

pH = - log(6.3 x 10^-8) $\pm$1

      = 7.2 $\pm$1

The buffering capacity range of H₂PO₄^-/HPO₄^2- is (7.2 - 1) = 6.2 to (7.2 + 1) = 8.2.

For HPO₄^2-/PO₄^3-,

pH = - log(4.2 x 10^-13) $\pm$1

      = 12.4 $\pm$1

The buffering capacity range of HPO₄^2-/PO₄^3- is (12.4 - 1) = 11.4 to (12.4 + 1) = 13.4.

Thus, the pH of 7 falls in the buffer capacity range of H₂PO₄^-/HPO₄^2-.

Hence, the best acid for the job = H₂PO₄^-/HPO₄^2-